Vinylpyrrolidone polymer and its stabilization and preservation processes

ABSTRACT

The present invention provides: a vinylpyrrolidone polymer which exhibits good heat resistance and storage stability; a composition containing the vinylpyrrolidone polymer; a stabilization process for the vinylpyrrolidone polymer, and a preservation process for a vinylpyrrolidone polymer by which: even when the vinylpyrrolidone polymer is preserved for a long time or at high temperature, the physical properties such as molecular weight (K value) of the vinylpyrrolidone polymer can be prevented from changing, therefore the vinylpyrrolidone polymer can stably be preserved. To enhance the heat resistance and the storage stability, a vinylpyrrolidone polymer is mixed with a certain amount of antioxidant, and the oxygen concentration is suppressed to not higher than 50,000 ppm in a gas phase that contacts with the vinylpyrrolidone polymer when preserving.

BACKGROUND OF THE INVENTION

[0001] A. Technical Field

[0002] The present invention relates to: a vinylpyrrolidone polymerwhich is excellent in the storage stability; a composition containingthe vinylpyrrolidone polymer; a stabilization process for thevinylpyrrolidone polymer; and a preservation process for thevinylpyrrolidone polymer.

[0003] B. Background Art

[0004] A vinylpyrrolidone polymer, such as poly(vinylpyrrolidone) or avinylpyrrolidone copolymer, is widely used in various fields such asmedicines, cosmetics, pressure sensitive adhesives or adhesives, paints,dispersants, inks, and electronic parts, because the vinylpyrrolidonepolymer has merits or advantages of biocompatibility, safety,hydrophilicity, and so on.

[0005] However, while providing the vinylpyrrolidone polymer to theabove various uses, the present applicant found that thevinylpyrrolidone polymer had problems in respect to the stability ofstorability and so on. For example, whether the vinylpyrrolidone polymeris a powder or solution, there were cases where the vinylpyrrolidonepolymer or a solution thereof got colored or underwent the reduction ofthe molecular weight or viscosity when stored for a long time or at hightemperature.

SUMMARY OF THE INVENTION

[0006] A. Object of the Invention

[0007] An object of the present invention is to provide: avinylpyrrolidone polymer which has extremely good storage stability andthermal stability in that even if the vinylpyrrolidone polymer is storedfor a long time, coloring or other changes of physical properties occurlittle, and the thermal stability is also excellent, and the storage ina solution state is also possible; a composition containing thevinylpyrrolidone polymer; and a stabilization process for thevinylpyrrolidone polymer.

[0008] Another object of the present invention is to provide apreservation process for a vinylpyrrolidone polymer by which: even whenthe vinylpyrrolidone polymer is preserved for a long time or at hightemperature, the physical properties such as molecular weight (K value)of the vinylpyrrolidone polymer can be prevented from changing,therefore the vinylpyrrolidone polymer can stably be preserved.

[0009] B. Disclosure of the Invention

[0010] The present inventor diligently studied to solve the aboveproblems. As a result, the inventor found that the above problems couldbe solved if the vinylpyrrolidone polymer was mixed with a certainamount of antioxidant, and further that a process, comprising the stepof examining how much a K value, as defined by Fikentscher's equation,reduced when heating the vinylpyrrolidone polymer under prescribedconditions, was good as a process for exactly judging the storagestability of the vinylpyrrolidone polymer. The inventor also found thatthe above problems could be solved if the oxygen concentration in a gasphase that contacts with the vinylpyrrolidone polymer was kept to acertain amount or less when preserving. The present invention has beencompleted on the basis of these findings.

[0011] Therefore, a vinylpyrrolidone polymer, according to the presentinvention, contains at least one antioxidant in a ratio of 0.00001˜30weight %, preferably 0.001˜30 weight %, to the vinylpyrrolidone polymer.Another vinylpyrrolidone polymer, according to the present invention, isadjusted such that the reduction ratio of the K value defined byFikentscher's equation will be within 5% when the vinylpyrrolidonepolymer is subjected to a forcible test in which the vinylpyrrolidonepolymer is kept heated at 120° C. under normal pressure in air for 2hours. Yet another vinylpyrrolidone polymer, according to the presentinvention, is adjusted such that the reduction ratio of the K valuedefined by Fikentscher's equation will be within 5% when thevinylpyrrolidone polymer is subjected to a promotion test in which thevinylpyrrolidone polymer is kept heated at 50° C. in a solution statefor 14 days.

[0012] A composition for various uses, according to the presentinvention, comprises a vinylpyrrolidone polymer as a resin component,and further comprises at least one antioxidant in a ratio of 0.00001˜30weight %, preferably 0.001˜30 weight %, to the vinylpyrrolidone polymer.

[0013] A stabilization process for a vinylpyrrolidone polymer, accordingto the present invention, comprises the step of mixing thevinylpyrrolidone polymer with at least one antioxidant in a ratio of0.00001˜30 weight %, preferably 0.001˜30 weight %, to thevinylpyrrolidone polymer.

[0014] A preservation process for a vinylpyrrolidone polymer, accordingto the present invention, comprises the step of suppressing the oxygenconcentration to not higher than 50,000 ppm in a gas phase that contactswith the vinylpyrrolidone polymer.

[0015] These and other objects and the advantages of the presentinvention will be more fully apparent from the following detaileddisclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, a detailed description is given about a mode forcarrying out the present invention.

[0017] The vinylpyrrolidone polymer in the present invention is,specifically, poly(vinylpyrrolidone) and/or a vinylpyrrolidonecopolymer.

[0018] The poly(vinylpyrrolidone) is a compound of general formula (3)below:

[0019] (wherein n is an integer)

[0020] and is obtained by polymerizing vinylpyrrolidone by any method asmentioned below.

[0021] The vinylpyrrolidone copolymer is a copolymer of which theconstituents include vinylpyrrolidone and a comonomer copolymerizabletherewith. The comonomer to be copolymerized with vinylpyrrolidone isnot especially limited, but specific examples thereof include: 1)(meth)acrylate esters such as methyl (meth)acrylate, ethyl(meth)acrylate, butyl (meth)acrylate, cyclohexyl (meth)acrylate, andhydroxyethyl (meth)acrylate; 2) (meth)acrylamide and derivativestherefrom such as N-monomethyl(meth)acrylamide,N-monoethyl(meth)acrylamide and N,N-dimethyl(meth)acrylamide; 3) basicunsaturated monomers such as dimethylaminoethyl (meth)acrylate,dimethylaminoethyl(meth)acrylamide, vinylpyridine, and vinylimidazoleand their salts or quaternized products; 4) vinylamides such asvinylformamide, vinylacetamide, and vinyloxazolidone; 5)carboxyl-group-containing unsaturated monomers such as (meth)acrylicacid, itaconic acid, maleic acid, and fumaric acid; 6) unsaturatedanhydrides such as maleic anhydride and itaconic anhydride; 7) vinylesters such as vinyl acetate and vinyl propionate; 8) vinylethylenecarbonate and derivatives therefrom; 9) styrene and derivativestherefrom; 10) 2-sulfoethyl (meth)acrylate and derivatives therefrom;11) vinylsulfonic acid and derivatives therefrom; 12) vinyl ethers suchas methyl vinyl ether, ethyl vinyl ether, and butyl vinyl ether; and 13)olefins such as ethylene, propylene, octene, and butadiene. Among thesegroups of comonomers, particularly, groups 1)˜8) are favorable, forexample, in respect to the copolymerizability with vinylpyrrolidone. Asto the above comonomers, only one kind may be copolymerized withvinylpyrrolidone, or any mixture of two or more kinds of comonomers maybe copolymerized with vinylpyrrolidone.

[0022] The ratio of vinylpyrrolidone to the entire monomer components ofthe vinylpyrrolidone copolymer is not especially limited, but ispreferably not lower than 0.1 mol %, more preferably not lower than 1.0mol %, still more preferably not lower than 5.0 mol %, most preferablynot lower than 20.0 mol %. In the case where the ratio ofvinylpyrrolidone in the copolymer is lower than 0.1 mol %, there aredisadvantages in that various properties deriving from vinylpyrrolidonecannot be exhibited and the deterioration of the polymer to be solved bythe present invention is seen little.

[0023] The polymerization reaction to give the vinylpyrrolidone polymercan be carried out by conventional polymerization methods such as bulkpolymerization, solution polymerization, emulsion polymerization,suspension polymerization, and precipitation polymerization. Thereaction temperature in the polymerization reaction may fitly be setaccording to conditions such as reactants, but is preferably in therange of 0˜250° C., more preferably 20˜150° C., most preferably 40˜100°C.

[0024] The polymerization reaction to give the vinylpyrrolidone polymermay be carried out by adding conventional polymerization initiators, ofwhich the specific examples include: radical polymerization initiatorssuch as azo compounds (e.g. 2,2′-azobisisobutyronitrile and2,2′-azobis(2-methylpropionamidine) dihydrochloride) and peroxides (e.g.benzoyl peroxide and hydrogen peroxide); and cationic polymerizationinitiators such as boron trifluoride or complexes thereof, iron(II)chloride, diethylaluminum chloride, diethylzinc, heteropolyacids, andactivated clay. The concentration of the polymerization initiator in thepolymerization reaction is not especially limited, but is preferably inthe range of 0.001˜10 weight %, more preferably 0.01˜5 weight %, mostpreferably 0.05˜3 weight %, of the monomer components. When thepolymerization reaction is carried out, for example, any chain transferagent, pH-adjusting agent, or buffer agent may fitly be used besides thepolymerization initiator as the need arises.

[0025] The solvent, as used for the polymerization reaction to give thevinylpyrrolidone polymer, or as used when preserving thevinylpyrrolidone polymer in a solution state, is not especially limited,but examples thereof include: water; alcohols such as methyl alcohol,ethyl alcohol, isopropyl alcohol, diethylene glycol; alkylene glycolethers (acetates) such as propylene glycol monomethyl acetate anddiethylene glycol monomethyl ether acetate; amides such asdimethylformamide and N-methylpyrrolidone; esters such as ethyl acetate,butyl acetate, and γ-butyrolactone; aliphatic hydrocarbons such ashexane and octane; alicyclic saturated hydrocarbons such as cyclohexane;alicyclic unsaturated hydrocarbons such as cyclohexene; aromatichydrocarbons such as benzene, toluene, and xylene; ketones such asacetone and methyl ethyl ketone; halogenated hydrocarbons such asdichloroethane, chloroform, and carbon tetrachloride; ethers such asdiethyl ether, dioxane, and tetrahydrofuran; sulfonate esters such asdimethyl sulfoxide; carbonate esters such as dimethyl carbonate anddiethyl carbonate; and alicyclic carbonate esters such as ethylenecarbonate and propylene carbonate. Among these, particularly, the ethers(acetates) and the amides are preferable, and water and the alcohols aremore preferable. These solvents may be used alone respectively or incombinations with each other. In addition, the above solvents arepreferably used such that the concentration of the monomer components inthe raw material mixture in the polymerization reaction can bepreferably in the range of 1˜99 weight %, more preferably 5˜70 weight %,most preferably 10˜60 weight %.

[0026] In the present invention, the stabilization of the properties,such as storability and heat resistance, of the vinylpyrrolidone polymeris designed by mixing the above vinylpyrrolidone polymer with at leastone antioxidant in a ratio of 0.00001˜30 weight %, preferably 0.001˜30weight %, to the vinylpyrrolidone polymer. In the case where the mixingratio of the antioxidant is lower than 0.00001 weight %, thestabilization of the properties such as storability and heat resistanceis difficult. In the case where the ratio is higher than 30 weight %,there are problems in that the inherent properties, such as strength andappearance, of the polymer are damaged.

[0027] Examples of the antioxidant usable in the present inventioninclude:

[0028] phenolic antioxidants such as sodium salicylate, potassium saltof methylbenzotriazole, 2-mercaptobenzimidazole,2,4-dihydroxybenzophenone, 2,6-di-t-butyl-p-cresol, butylatedhydroxyanisole, 2,6-di-t-butyl-4-methylphenol,2,6-di-t-butyl-4-ethylphenol, stearyl-β-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, propyl 3,4,5-trihydroxybenzoate, hydroquinone, and catechol;

[0029] bisphenolic antioxidants such as2,2′-methylenebis(4-methyl-6-t-butylphenol),2,2′-methylenebis(4-ethyl-6-t-butylphenol),4,4′-thiobis(3-methyl-6-t-butylphenol),4,4′-butylidenebis(3-methyl-6-t-butylphenol),3,9-bis[1,1-dimethyl-2-[β-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy]ethyl]2,4,8,10-tetraoxaspyrro[5,5]undecane, and4,4′-(2,3-dimethyl-tetramethylene)dipyrrocatechol;

[0030] high-molecular phenolic antioxidants such as1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane,1,3,5-trinethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene,tetrakis-[methylene-3-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methane, bis[3,3′-bis-(4′-hydroxy-3′-t-butylphenyl)butyricacid] glycol ester,1,3,5-tris(3′,5′-di-t-butyl-4′-hydroxybenzyl)-s-triazine-2,4,6-(1H, 3H,5H)trione, and tocopherol;

[0031] sulfur-containing antioxidants such as dilauryl3,3′-thiodipropionate, dimyristyl 3,3′-thiodipropionate, distearyl3,3′-thiodipropionate, 2-mercaptobenzimidazole,tetrakismethylene-3-(laurylthio)propionatemethane, andstearylthiopropylamide;

[0032] phosphorus-containing antioxidants such as triphenyl phosphite,diphenylisodecyl phosphite, phenyldiisodecyl phosphite,4,4′-butylidene-bis(3-methyl-6-t-butylphenylditridecyl) phosphite,cyclic neopentanetetraylbis(octadecyl) phosphite, tris(nonylphenyl)phosphite, tris(mono- and/or dinonylphenyl) phosphite,diisodecylpentaerythritol diphosphite,9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide,10-(3,5di-t-butyl-4-hydroxybenzyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide,10 decyloxy-9,10-dihydro-9-oxa-10-phosphaphenanthrene,tris(2,4-di-t-butylphenyl) phosphite, cyclicneopentanetetraylbis(2,4-di-t-butylphenyl) phosphite, cyclicneopentanetetraylbis(2,6-di-t-butyl-4-methylphenyl) phosphite,2,2-methylenebis(4,6-d 1-t-butylphenyl)octyl phosphite,distearylpentaerythritol diphophite, di(2,4-di-t-butylphenyl) phophite,and tetrakis(2,4-di-t-butylphenyl)-4,4-biphenylene phophonite;

[0033] alcoholic antioxidants such as erysorbic acid, sodium erysorbate,and isopropyl citrate;

[0034] amine antioxidants such as methylated diphenylamine, ethylateddiphenylamine, butylated diphenylamine, octylated diphenylamine,laurylated diphenylamine, N,N′-di-sec-butyl-p-phenylenediamine, andN,N′-diphenyl-p-phenylenediamine;

[0035] hindered-amine antioxidants such as4-benzoyloxy-2,2,6,6-tetramethylpiperidine,bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate,bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidinyl) sebacate,bis(1,2,2,6,6-pentamethyl-4-piperidinyl) sebacate, dimethylsuccinate-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidine andcondensed products thereof, and8-acetyl-3-dodecyl-7,7,9,9-tetramethyl-1,3,8-triazaspyrro[4,5]decane-2,4dione.These may be used alone respectively or in combinations with each other.

[0036] Among the above-exemplified antioxidants, particularly, compoundshaving a phenolic hydroxyl group are preferably used in the presentinvention, of which the specific examples include the phenolicantioxidants, the bisphenolic antioxidants, and the high-molecularphenolic antioxidants. If the compound having a phenolic hydroxyl groupis used as the antioxidant, there are advantages of not increasing theash content resultant from strong heating of the vinylpyrrolidonepolymer.

[0037] As to the antioxidant, particularly, compounds of thebelow-mentioned general formulae (1) and/or (2) are favorably usedbesides the above-exemplified antioxidants in the present invention,because the deterioration of the vinylpyrrolidone polymer is, forexample, considered to occur due to participation of radical speciesthat form by cleavage of peroxides that are generated by influence ofultraviolet rays and so on, but because the radical scavengers of thebelow-mentioned general formulae (1) and/or (2) make resonant structuresin a radicalized state, and therefore have so high radicalscavengeability as to be extremely excellent in the ability to preventthe polymer from deteriorating with the passage of time.

[0038] wherein each of R^(1, R) ², R³, R⁴, and R⁵ is, independently ofeach other, at least one member selected from the group consisting of ahydrogen atom, alkyl groups, substituted alkyl groups, aryl groups, andsubstituted aryl groups, wherein R¹ and R², or R³ and R⁴, may be bondedto each other to form a cyclic structure.

[0039] Specific examples of the compound of general formula (1) aboveinclude thioformamide, thioacetamide, thiopropionamide,N-methylthioformamide, N-methylthioacetamide, and N-ethylthioformamide,and specific examples of the compound of general formula (2) aboveinclude thiourea, N,N′-dimethylthiourea, N,N′-diethylthiourea,N,N′-dibutylthiourea, N,N′-diphenylthiourea, ethylenethiourea, andpropylenethiourea, but there is no limiation thereto. Among them,particularly, cases where R⁴ and/or R⁵ in formula (2) is a hydrogenatom, in other words, cases where the compound of this formula isthiourea or a derivative therefrom, are preferable because, in suchcases, the compound can make four kinds of resonant stabilizedstructures in a radicalized state and therefore has the highest radicalscavengeability. When these compounds of general formulae (1) and/or (2)are used, the amount thereof is preferably in the range of 0.00001 to 10weight %, more preferably 0.005 to 1 weight %, of the vinylpyrrolidonepolymer.

[0040] In the present invention, moreover, it is preferable that theradical scavenger is jointly used with a peroxide decomposer. The jointuse of the radical scavenger with the peroxide decomposer makessynergistic effects upon the prevention of the deterioration of thevinylpyrrolidone polymer, and therefore can maintain the effects for alonger time or at higher temperature than cases where they are usedalone respectively.

[0041] The above peroxide decomposer reduces either undecomposed residueof peroxides as used for producing the vinylpyrrolidone polymer, orperoxides which form due to factors such as oxidation of radical specieswhile the vinylpyrrolidone polymer is, for example, preserved ortransported. Specific examples of the peroxide decomposer include thosewhich are previously enumerated as the specific examples of the sulfur-or phosphorus-containing antioxidant among the above-exemplifiedantioxidants. When these peroxide decomposers are used, the amountthereof is preferably in the range of 0.00001 to 10 weight %, morepreferably 0.005 to 1 weight %, of the vinylpyrrolidone polymer.

[0042] The aforementioned radical scavenger scavenges radicals, whichare generated by influence of light, heat, and so on while thevinylpyrrolidone polymer is, for example, preserved or transported, withthe result that the radicals are prevented from being chained toframeworks of the polymer. Specific examples of the radical scavengerinclude, among the above-exemplified antioxidants, those which arepreviously enumerated as the specific examples of the phenolicantioxidant, the bisphenolic antioxidant, the high-molecular phenolicantioxidant, the alcoholic antioxidant, the amine antioxidant, and thehindered-amine antioxidant, and further, those which are previouslyenumerated as the specific examples of the compound of general formula(1) and/or (2). In the present invention, among these, particularly, thephenolic antioxidants, the bisphenolic antioxidants, the amineantioxidants, the hindered-amine antioxidants, and thiourea andderivatives therefrom are preferable, and thiourea and derivativestherefrom are more preferable. When these radical scavengers are used,the amount thereof is preferably in the range of 0.00001 to 10 weight %,more preferably 0.005 to 1 weight %, of the vinylpyrrolidone polymer.

[0043] In the present invention, it is preferable that thevinylpyrrolidone polymer further contains a radical formation inhibitor,because mixing of this radical formation inhibitor might make moresynergistic effects upon the prevention of the deterioration of thevinylpyrrolidone polymer. Specific examples of the radical formationinhibitor include conventional ultraviolet absorbent, metal-deactivatingagents, and quenchers. When these radical formation inhibitors are used,the amount thereof is preferably in the range of 0.00001 to 10 weight %of the vinylpyrrolidone polymer.

[0044] In the present invention, the method for mixing thevinylpyrrolidone polymer with the antioxidant (including the radicalscavenger and the peroxide decomposer) and further, as the need arises,with the radical formation inhibitor is not especially limited, but, ina preferable method, these additives are added to a reaction liquidresultant from the polymerization reaction, or to a solution ordispersion of the vinylpyrrolidone polymer which is obtained bydissolving or dispersing a powder of the vinylpyrrolidone polymer into asolvent.

[0045] When the vinylpyrrolidone polymer according to the presentinvention is formed into an aqueous solution with a concentration of 5weight %, its pH is preferably in the range of 4 to 12, more preferably5 to 9. In these pH regions, the stabilization effect due to theantioxidant in the present invention can be exhibited still moreeffectively. In the case where pH is on the more acidic side than 4, theoxidation deterioration easily occurs, therefore the durability of theaforementioned stabilization effect tends to be damaged.

[0046] According to the present invention, the storage stability of thevinylpyrrolidone polymer can surely be judged by the following process.The vinylpyrrolidone polymer or a composition, which is prepared bymixing the vinylpyrrolidone polymer with a storage-stabilizing agentsuch as antioxidant, is subjected to a forcible test in which thevinylpyrrolidone polymer or the composition is kept heated at 120° C.under normal pressure in air for 2 hours. Before and after this forcibletest, the vinylpyrrolidone polymer is dissolved into any solvent, inwhich the vinylpyrrolidone polymer is soluble, in a concentration of nothigher than 10 weight %. The respective viscosities of the resultantsolutions are measured at 25° C. with a capillary viscometer. RespectiveK values are determined from the resultant measured values, wherein theK values are defined by Fikentscher's equation. Incidentally, in thecase where the judgment of the storage stability of the vinylpyrrolidonepolymer is carried out in a solution state, the vinylpyrrolidone polymeris subjected to a promotion test in which the vinylpyrrolidone polymeris kept heated at 50° C. in a solution state for 14 days, and then thesolution is diluted with any solvent to decrease the content of thevinylpyrrolidone polymer to not higher than 10 weight %, and then theviscosity of the resultant solution is measured to determine the Kvalue. Then, either the reduction ratio of the K value after theforcible test to the initial value of the K value before the forcibletest or the reduction ratio of the K value after the promotion test tothe initial value of the K value before the promotion test is evaluated.As a result, the storage stability of the vinylpyrrolidone polymer caneasily be judged. As to the vinylpyrrolidone polymer (including thecomposition as prepared by mixing the vinylpyrrolidone polymer with astorage-stabilizing agent such as antioxidant) which has excellentstorage stability, the above reduction ratio is suppressed within 5%.

[0047] Herein, the K value is a value which is calculated from themeasured viscosity and the following Fikentscher's equation:

(log η _(rel))/C=[(75K ₀ ²)/(1+1.5K ₀ C)]+K ₀

[0048] K=1000K₀

[0049] wherein: C denotes the grams of the vinylpyrrolidone polymer in100 ml of solution; and η_(rel) denotes the viscosity of the solution tothe solvent.

[0050] As to the storage-stabilized vinylpyrrolidone polymer accordingto the present invention (i.e. the vinylpyrrolidone polymer which isadjusted such that the reduction ratio of the K value defined byFikentscher's equation will be within 5% when the vinylpyrrolidonepolymer is subjected to a forcible test in which the vinylpyrrolidonepolymer is kept heated at 120° C. under normal pressure in air for 2hours or to a promotion test in which the vinylpyrrolidone polymer iskept heated at 50° C. in a solution state for 14 days), its storagestabilization process by other than mixing with the aforementionedantioxidant is also adoptable providing that it involves mixing with thestorage-stabilizing agent or storage stabilization process by which theabove reduction ratio of the K value can be maintained.

[0051] In the present invention, as the need arises, thevinylpyrrolidone polymer may be allowed to contain, for example, variousadditives such as processing stabilizers, plasticizers, dispersants,fillers, aging inhibitors, pigments, and curing agents.

[0052] In the preservation process of the present invention, it isimportant to suppress the oxygen concentration to not higher than 50,000ppm in a gas phase that contacts with the vinylpyrrolidone polymer. Theoxygen concentration is preferably suppressed to not higher than 10,000ppm, more preferably not higher than 1,000 ppm. The lower the oxygenconcentration is, the more effectively the preservation stability of thevinylpyrrolidone polymer can be enhanced. In the case where the oxygenconcentration is higher than 50,000 ppm, the physical properties such asmolecular weight (K value) of the vinylpyrrolidone polymer change withthe passage of time when the vinylpyrrolidone polymer is preserved for along time or at high temperature. When applying the preservation processof the present invention, product form of the vinylpyrrolidone polymeris not especially limited, but may be any form, for example, lumps,fibers, powders, solutions, films.

[0053] Herein, the gas phase is a portion other than a solid phaseportion and/or a liquid phase portion as occupied by thevinylpyrrolidone polymer. The oxygen concentration in the gas phasemeans the absolute amount of oxygen as contained in a gas phase of aunit volume. In the case where the vinylpyrrolidone polymer is put in anairtight container, the oxygen concentration in the gas phase is a ratioof a volume as occupied by oxygen of the gas phase portion in thecontainer under normal pressure to a volume of the gas phase portion.Incidentally, the oxygen concentration, for example, can easily bemeasured with commercially available oxygen analyzers such as galvaniccell diffusion types or zirconia sensor types.

[0054] A preferable example of a first means for suppressing the oxygenconcentration in a gas phase that contacts with the vinylpyrrolidonepolymer is a process comprising the steps of putting thevinylpyrrolidone polymer into an airtight container and evacuating thecontainer in the preservation process of the present invention.

[0055] The specific method for evacuating the container is notespecially limited, and the container may be evacuated in conventionalmanners. In addition, the vacuum degree is not especially limited,either, but it is preferable to achieve the vacuum such that thecontainer may not be broken.

[0056] A preferable example of a second means for suppressing the oxygenconcentration in a gas phase that contacts with the vinylpyrrolidonepolymer is a process comprising the steps of putting thevinylpyrrolidone polymer into an airtight container and displacing airin the container with an inert gas and/or carbonic acid gas in thepreservation process of the present invention. Particularly, it is morepreferable that the displacement of air in the container (in which thevinylpyrrolidone polymer is put) is carried out with carbonic acid gas,because the preservation stability of the vinylpyrrolidone polymer isenhanced, and because the content of unreacted vinylpyrrolidoneremaining in the vinylpyrrolidone polymer can be reduced by hydrolysisduring the preservation. Incidentally, it is preferable that thedisplacement with an inert gas and/or carbonic acid gas is carried outnot only after the vinylpyrrolidone polymer has been put into thecontainer, but also in the step of transferring the vinylpyrrolidonepolymer from a reservoir therefor to the airtight container. Forexample, it is preferable to keep conditions such as the reservoir or atransferring line in a state filled with the aforementioned gas.

[0057] The aforementioned inert gas is a gas that is poor in thereactivity, and specific examples thereof include: gases on group 0 inthe periodic table, such as helium, neon, and argon; and nitrogen gas.Among these, nitrogen gas is preferable because it is easily available.

[0058] The specific method to displace air in the container with aninert gas and/or carbonic acid gas is not especially limited, but it ispreferable that the volume of the gas, as introduced into the containerin the displacement step with the gas, is at least 5 times as large asthe capacity of the container. In addition, it is preferable that theintroduction of the gas into the container is carried out after removingair by once evacuating the container, because the displacement with thegas can thereby more efficiently be carried out. Furthermore, in thecase where the vinylpyrrolidone polymer is a solution, it is preferableto allow the solution to bubble by using a tool, such as agas-introducing tube, in order to also remove oxygen as dissolved in thesolution.

[0059] A preferable example of a third means for suppressing the oxygenconcentration in a gas phase that contacts with the vinylpyrrolidonepolymer is a process comprising the steps of putting thevinylpyrrolidone polymer into an airtight container and enclosing atleast one deoxidizing agent together with the vinylpyrrolidone polymerin the container in the preservation process of the present invention.

[0060] The deoxidizing agent is not especially limited if it can removeoxygen by a chemical reaction. However, in view of the safety or theeasy use, for example, those which are obtained by sealing an inorganicdeoxidizing agent such as an iron powder, iron oxide, iron hydroxide, oran organic deoxidizing agent such as L-ascorbic acid, or any mixturethereof in with gas-permeable films are preferable. A moisture keepingagent to facilitate deoxidizing can be sealed together. Specifically,commercially available deoxidizing agents, such as “Ageless” made byMitsubishi Gas Chemical Co., Inc., “Bitaron” made by Toagosei Co., Ltd.,and “Modulan” made by Nippon Kayaku Co., Ltd., may be used.

[0061] In the aforementioned respective means for suppressing the oxygenconcentration, the container to put the vinylpyrrolidone polymer into isnot especially limited by the material or shape of the container if itcan achieve the airtightness. However, containers which are so highairtight as to involve little gas absorption or leakage are preferable.Specifically, examples of the material of the container include glass,metals, and various plastics, and examples of the shape of the containerinclude bottles, cans, and bags.

[0062] In the preservation process of the present invention, the first,second, and third means, which are aforementioned as the process forsuppressing the oxygen concentration in a gas phase that contacts withthe vinylpyrrolidone polymer, are used alone respectively or,preferably, in combinations with each other, whereby the oxygenconcentration can effectively be suppressed.

[0063] It is preferable that the preservation process according to thepresent invention is applied also to the steps ranging from after theproduction of the vinylpyrrolidone polymer to the transfer of thevinylpyrrolidone polymer into a preservation container. For example, itis preferable to suppress the oxygen concentration to not higher thanthe aforementioned certain concentration in the gas phase in places suchas reaction apparatuses, powdering apparatuses, drying apparatuses,reservoirs, and transferring lines just after the production.

[0064] The preservation stability of the vinylpyrrolidone polymerpreserved by the preservation process of the present invention, forexample, can be judged from the reduction ratio between K values beforeand after a forcible test as measured by the following process. Thevinylpyrrolidone polymer, which is preserved in an airtight container,is subjected to a forcible test in which the vinylpyrrolidone polymer iskept heated together with the container at 120° C. for 2 hours. Beforeand after this forcible test, the vinylpyrrolidone polymer is dissolvedinto any solvent, in which the vinylpyrrolidone polymer is soluble, in aconcentration of not higher than 10 weight %. The respective viscositiesof the resultant solutions are measured at 25° C. with a capillaryviscometer. Respective K values are determined from the resultantmeasured values, wherein the K values are defined by Fikentscher'sequation. The reduction ratio of the K value after the forcible test tothe initial value of the K value before the forcible test is evaluated.As a result, the preservation stability of the vinylpyrrolidone polymercan easily be judged. If the resultant reduction ratio is within 5%, thevinylpyrrolidone polymer can stably be preserved without any change ofthe physical properties even for a long time or at high temperature.

[0065] The initial value of the K value of the vinylpyrrolidone polymer,to which the present invention is applicable, is not especially limited,but the present invention is preferably applied to a vinylpyrrolidonepolymer having a K value in the range of 15˜150, preferably 15˜120. Inaddition, because particularly the stability of a vinylpyrrolidonepolymer having a high K value is inferior, the present invention is morepreferably applied to a vinylpyrrolidone polymer having a K value in therange of 60˜150, preferably 60˜120.

[0066] The storage-stabilized vinylpyrrolidone polymer and preservedvinylpyrrolidone polymer according to the present invention are usablefor vinylpyrrolidone-polymer-containing compositions that are used invarious fields such as medicines, cosmetics, pressure sensitiveadhesives or adhesives, paints, dispersants, inks, and electronic parts.In addition, such a vinylpyrrolidone polymer composition, according tothe present invention, means not only a composition containing thepresent invention vinylpyrrolidone polymer which is beforehand mixedwith the storage-stabilizing agent such as antioxidant or of which the Kvalue is beforehand adjusted, but also a composition which is notbeforehand mixed with the storage-stabilizing agent, but to which thereis separately added a material (such as antioxidant) that contributes tothe storage stabilization of the vinylpyrrolidone polymer as things turnout.

[0067] (Effects and Advantages of the Invention):

[0068] The present invention can provide a vinylpyrrolidone polymer,wherein the vinylpyrrolidone polymer has the K value that is preventedfrom reducing, therefore this vinylpyrrolidone polymer exhibits goodheat resistance and storage stability.

[0069] The present invention can further provide a composition whichcontains the vinylpyrrolidone polymer and therefore exhibits good heatresistance and storage stability.

[0070] The present invention can further provide a stabilization processby which the heat resistance and the storage stability of thevinylpyrrolidone polymer are enhanced.

[0071] By the present invention, even when the vinylpyrrolidone polymeris preserved for a long time or at high temperature, the physicalproperties such as molecular weight (K value) of the vinylpyrrolidonepolymer can be prevented from changing, therefore the vinylpyrrolidonepolymer can stably be preserved. In addition, in the case where air in acontainer in which the vinylpyrrolidone polymer is put is displaced withcarbonic acid gas, the content of vinylpyrrolidone which remains as amonomer in the vinylpyrrolidone polymer can be reduced by hydrolysisduring the preservation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0072] Hereinafter, the present invention is more specificallyillustrated by the following examples of some preferred embodiments incomparison with comparative examples not according to the invention.However, the present invention is not limited to the below-mentionedexamples.

[0073] The K values in the examples and the comparative examples werecalculated in the following way: the viscosity was measured by theaforementioned viscosity measurement process, that is, a process inwhich the relative viscosity of a 0.1˜2 weight % aqueous solution ofeach vinylpyrrolidone polymer of the examples and the comparativeexamples was measured at 25° C. with a capillary viscometer; and theresultant measured viscosity was applied to Fikentscher's equation asaforementioned.

EXAMPLE 1

[0074] First, 10 g of poly(vinylpyrrolidone) (weight-average molecularweight=980,000; initial value of K value=86) and 0.1 g of sodiumsalicylate, as an antioxidant, were dissolved into 40 g of water. Theresultant aqueous solution was cast onto a Teflon sheet, and then driedat 90° C. under a vacuum of 6,700 Pa (50 mmHg), and then pulverized,thus obtaining a poly(vinylpyrrolidone) powder. This powder wassubjected to a forcible test in which the powder was heated at 120° C.under normal pressure in air for 2 hours. The resultant resincomposition had a weight-average molecular weight of 970,000, and the Kvalue of this composition was 82, therefore the reduction ratio of the Kvalue was not higher than 5%.

COMPARATIVE EXAMPLE 1

[0075] The procedure was carried out in the same way as of Example 1except that no sodium salicylate was added. The resultant resincomposition had a weight-average molecular weight of 650,000, and the Kvalue reduced to 69 (reduction ratio=20%).

EXAMPLE 2

[0076] The procedure was carried out in the same way as of Example 1except that: 10 g of poly(vinylpyrrolidone) (weight-average molecularweight=1,050,000; initial value of K value=91) and 0.05 g of sodiumsalicylate, as an antioxidant, were dissolved into 40 g of water, andthen, to the resultant aqueous solution, 0.1 g of2,2′-azobis(2-methylpropionamidine) dihydrochloride was added. Theresultant resin composition had a weight-average molecular weight of1,050,000, and the K value of this composition was 91, therefore therewas no change of the K value from its initial value.

COMPARATIVE EXAMPLE 2

[0077] The procedure was carried out in the same way as of Example 2except that no sodium salicylate was added. The resultant resincomposition had a weight-average molecular weight of 670,000, and the Kvalue reduced to 69 (reduction ratio=24%).

EXAMPLE 3

[0078] The procedure was carried out in the same way as of Example 1except that sodium salicylate was replaced with potassium salt ofmethylbenzotriazole. The resultant resin composition had aweight-average molecular weight of 960,000, and the K value of thiscomposition was 83, therefore the reduction ratio of the K value was nothigher than 5%.

EXAMPLE 4

[0079] The procedure was carried out in the same way as of Example 1except that sodium salicylate was replaced with 0.01 g of hydroquinone.The resultant resin composition had a weight-average molecular weight of970,000, and the K value of this composition was 85, therefore thereduction ratio of the K value was not higher than 5%.

EXAMPLE 5

[0080] First, 10 g of poly(vinylpyrrolidone) (weight-average molecularweight=850,000; initial value of K value=80) and 0.1 g of2,6-di-t-butyl-4-methylphenol, as an antioxidant, were dissolved into 40g of methanol. The resultant solution was cast onto a Teflon sheet, andthen dried at 70° C. under a vacuum of 6,700 Pa (50 mmHg), and thenpulverized, thus obtaining a poly(vinylpyrrolidone) powder. Theresultant poly(vinylpyrrolidone) powder was subjected to the sameforcible test as of Example 1. The resultant resin composition had aweight-average molecular weight of 830,000, and the K value of thiscomposition was 79, therefore the reduction ratio of the K value was nothigher than 5%.

EXAMPLE 6

[0081] The procedure was carried out in the same way as of Example 5except that 2,6-di-t-butyl-4-methylphenol was replaced with2-mercaptobenzimidazole. The resultant resin composition had aweight-average molecular weight of 820,000, and the K value of thiscomposition was 78, therefore the reduction ratio of the K value was nothigher than 5%.

EXAMPLE 7

[0082] The procedure was carried out in the same way as of Example 5except that 2,6-di-t-butyl-4-methylphenol was replaced with triphenylphosphite. The resultant resin composition had a weight-averagemolecular weight of 800,000, and the K value of this composition was 76,therefore the reduction ratio of the K value was not higher than 5%.

EXAMPLE 8

[0083] First, 10 g of a copolymer of vinylpyrrolidone/vinylacetate=80/20 (molar ratio) (weight-average molecular weight=610,000;initial value of K value=50) and 0.01 g of hydroquinone, as anantioxidant, were dissolved into 40 g of water. The resultant aqueoussolution was cast onto a Teflon sheet, and then dried at 90° C. under avacuum of 6,700 Pa (50 mmHg), and then pulverized, thus obtaining avinylpyrrolidone/vinyl acetate copolymer powder. The resultantvinylpyrrolidone/vinyl acetate copolymer powder was subjected to thesame forcible test as of Example 1. The resultant resin composition hada weight-average molecular weight of 590,000, and the K value of thiscomposition was 49, therefore the reduction ratio of the K value was nothigher than 5%.

COMPARATIVE EXAMPLE 3

[0084] The procedure was carried out in the same way as of Example 8except that no hydroquinone was added. The resultant resin compositionhad a weight-average molecular weight of 500,000, and the K valuereduced to 42 (reduction ratio=16%).

EXAMPLE 9

[0085] The procedure was carried out in the same way as of Example 4except that the amount of hydroquinone was changed to 0.0005 g. Theresultant resin composition had a weight-average molecular weight of920,000, and the K value of this composition was 82, therefore thereduction ratio of the K value was not higher than 5%.

COMPARATIVE EXAMPLE 4

[0086] The procedure was carried out in the same way as of Example 4except that the amount of hydroquinone was changed to 0.00003 g. Theresultant resin composition had a weight-average molecular weight of880,000, and the K value reduced to 88 (reduction ratio=7%).

EXAMPLE 10

[0087] The procedure was carried out in the same way as of Example 1except that the amount of sodium salicylate was changed to 1.0 g. Theresultant resin composition had a weight-average molecular weight of980,000, and the K value of this composition was 85, therefore thereduction ratio of the K value was not higher than 5%.

EXAMPLE 11

[0088] The procedure was carried out in the same way as of Example 1except that sodium salicylate was replaced with 0.01 g of thiourea. Theresultant resin composition had a weight-average molecular weight of970,000, and the K value of this composition was 85, therefore thereduction ratio of the K value was not higher than 5%.

EXAMPLE 12

[0089] First, 10 g of poly(vinylpyrrolidone), having a K value of 88,and 0.005 g of thiourea were dissolved into 40 g of water. The resultantaqueous solution was subjected to a promotion test in which the aqueoussolution was heated at 50° C. for 14 days. As a result, the K value ofthe aqueous poly(vinylpyrrolidone) solution was 87 after the promotiontest, therefore the reduction ratio of the K value was not higher than5%.

COMPARATIVE EXAMPLE 5

[0090] The procedure was carried out in the same way as of Example 12except that no thiourea was used. As a result, the K value of theaqueous poly(vinylpyrrolidone) solution reduced to 74 by the promotiontest (reduction ratio 16%).

EXAMPLE 13

[0091] First, 50 g of poly(vinylpyrrolidone) (initial value of K value=88) and 0.05 g of thiourea, as an antioxidant, were dissolved into 200g of water. The resultant aqueous solution had a pH of 6.5. This aqueoussolution was cast onto a Teflon sheet, and then dried at 90° C. under avacuum of 6,700 Pa (50 mmHg), and then pulverized, thus obtaining apoly(vinylpyrrolidone) powder. This powder was kept at 50° C. in air,and the K value was measured every 7 days to evaluate the durability ofthe K value stability by the number of the days that had passed untilthe reduction ratio of the K value to its initial value increased to notlower than 5% (K value stability duration (days)). As a result, the Kvalue stability duration was 70 days.

EXAMPLES 14 TO 16

[0092] Each procedure was carried out in the same way as of Example 13except that thiourea was replaced with each antioxidant as shown inTable 1. The K value stability duration (days) of each resultantpoly(vinylpyrrolidone) powder is shown in Table 1.

EXAMPLES 17 TO 25

[0093] First, 50 g of poly(vinylpyrrolidone) (initial value of Kvalue=88) and 0.05 g of each antioxidant, as shown in Table 1, weredissolved into 200 g of methanol. The resultant solution was cast onto aTeflon sheet, and then dried at 70° C. under a vacuum of 6,700 Pa (50mmHg), and then pulverized, thus obtaining each poly(vinylpyrrolidone)powder. The K value stability duration (days), as determined in the sameway as of Example 13, of each resultant poly(vinylpyrrolidone) powder isshown in Table 1.

COMPARATIVE EXAMPLE 6

[0094] The procedure was carried out in the same way as of Example 13except that no antioxidant was used. The K value stability duration(days) of the resultant poly(vinylpyrrolidone) powder is shown in Table1.

EXAMPLE 26

[0095] The procedure was carried out in the same way as of Example 13except that 0.005 g of succinic acid was used to decrease the pH of theaqueous solution to 5.5. The K value stability duration (days) of theresultant poly(vinylpyrrolidone) powder is shown in Table 1.

EXAMPLE 27

[0096] The procedure was carried out in the same way as of Example 13except that 0.005 g of sodium hydroxide was used to increase the pH ofthe aqueous solution to 11.6. The K value stability duration (days) ofthe resultant poly(vinylpyrrolidone) powder is shown in Table 1.

COMPARATIVE EXAMPLE 7

[0097] The procedure was carried out in the same way as of Example 13except that 0.01 g of succinic acid was used to decrease the pH of theaqueous solution to 3.5. The K value stability duration (days) of theresultant poly(vinylpyrrolidone) powder is shown in Table 1. TABLE 1 Kvalue stability Antioxidant duration (days) Example 13 Thiourea 70Example 14 Dimethylthiourea 56 Example 15 Thioacetamide 49 Example 16Hydroquinone 56 Example 17 4-Benzoyloxy-2,2,6,6- 49tetramethylpiperidine Example 18 N,N′-di-sec- 42 butylphenylenediamineExample 19 2-Mercaptobenzimidazole 42 Example 20 Triphenyl phosphite 35Example 21 Thiourea 196  2-Mercaptobenzimidazole Example 22Thioacetamide 175  2-Mercaptobenzimidazole Example 234-Benzoyloxy-2,2,6,6- 175  tetramethylpiperidine 2-MercaptobenzimidazoleExample 24 N,N′-di-sec- 119  butylphenylenediamine Triphenyl phosphiteExample 25 Hydroquinone 168  Triphenyl phosphite Example 26 Thiourea 56Succinic acid (pH 5.5) Example 27 Thiourea 70 Sodium hydroxide (pH 11.6)Comparative — 14 Example 6 Comparative Thiourea 21 Example 7 Succinicacid (pH 3.5)

EXAMPLE 28

[0098] First, 100 g of powdery poly(vinylpyrrolidone) having a K valueof 91 was put into a glass container of 500 ml in capacity with anairtightenable lid and an oxygen sensor, and the glass container wasthen 10 set in a glove box. Then, nitrogen gas was introduced into theglove box for about 30 minutes to suppress the oxygen concentration inthe glove box to 20 ppm, and the glass container was then airtightenedto enclose the powder of poly(vinylpyrrolidone) with nitrogen gas.

[0099] This poly(vinylpyrrolidone) was heated together with thecontainer by an oven of 120° C. for 2 hours, and then cooled to roomtemperature. The K value of the poly(vinylpyrrolidone) was 91 after theheating treatment, therefore it was confirmed that the K value had notchanged.

EXAMPLE 29

[0100] First, 300 g of 20% aqueous solution of poly(vinylpyrrolidone)having a K value of 91 was put into a glass container of 500 ml incapacity with an airtightenable lid and an oxygen sensor, and the glasscontainer was then set in a glove box. Then, nitrogen gas was introducedinto the glove box for about 30 minutes to suppress the oxygenconcentration in the glove box to 20 ppm, and simultaneously, thesolution in the container was allowed to bubble with nitrogen gas at aflow rate of 100 ml/minute for 30 minutes by using anitrogen-gas-introducing tube having a gas filter on the tip.Thereafter, the glass container was then airtightened to enclose theaqueous poly(vinylpyrrolidone) solution with nitrogen gas.

[0101] This poly(vinylpyrrolidone) was subjected to the same heatingtreatment as of Example 28. As a result, the K value of thepoly(vinylpyrrolidone) was 91 after the heating treatment, therefore itwas confirmed that the K value had not changed.

EXAMPLE 30

[0102] First, 300 g of 20% aqueous solution of poly(vinylpyrrolidone)having a K value of 87 (containing vinylpyrrolidone in a ratio of 50ppm) was put into a glass container of 500 ml in capacity with anairtightenable lid and an oxygen sensor, and the glass container wasthen set in a glove box. Then, carbonic acid gas was introduced into theglove box for about 30 minutes to suppress the oxygen concentration inthe glove box to 20 ppm, and simultaneously, the solution in thecontainer was allowed to bubble with carbonic acid gas at a flow rate of50 ml/minute for 30 minutes by using a carbonic-acid-gas-introducingtube having a gas filter on the tip. Thereafter, the glass container wasthen airtightened to enclose the aqueous poly(vinylpyrrolidone) solutionwith carbonic acid gas. Incidentally, pH of the solution was 3.9 afterthe carbonic acid gas bubbling.

[0103] This poly(vinylpyrrolidone) was subjected to the same heatingtreatment as of Example 28. As a result, the K value of thepoly(vinylpyrrolidone) was 86 after the heating treatment, therefore itwas confirmed that the K value had changed little (reductionratio=1.1%).

[0104] In addition, the above poly(vinylpyrrolidone) was preserved at25° C. for 1 month in the state enclosed with carbonic acid gas, andthen sampled in the glove box to quantify vinylpyrrolidone in thesolution by liquid chromatography. As a result, it was confirmed thatthe vinylpyrrolidone content had reduced to not higher than 1 ppm.

EXAMPLE 31

[0105] First, 20 g of powdery poly(vinylpyrrolidone) having a K value of89 was put into a heat-sealable PET bag. Then, an oxygen sensor and aglass tube having a gas filter on the tip were introduced into the bag,and the other portions of the bag were airtightened by heat-sealing.Then, the glass tube was connected to a vacuum system of 50 mmHg to suckair out of the bag. After 15 minutes, the glass tube was disconnectedfrom the vacuum system and then quickly sealed tightly to enclose thepoly(vinylpyrrolidone) powder in a vacuum state. At that time, theoxygen concentration in the bag was 180 ppm.

[0106] This poly(vinylpyrrolidone) was subjected to the same heatingtreatment as of Example 28 together with the bag. As a result, the Kvalue of the poly(vinylpyrrolidone) was 88 after the heating treatment,therefore it was confirmed that the K value had changed little(reduction ratio=1.1%).

EXAMPLE 32

[0107] First, 20 g of powdery poly(vinylpyrrolidone) having a K value of91 was put into a glass container of 100 ml in capacity with anairtightenable lid and an oxygen sensor. Then, “Ageless SA” made byMitsubishi Gas Chemical Co., Inc. was fixed onto the back side of thelid, and then the container was airtightened by closing the lid and leftin this state for 24 hours. After this leaving, the oxygen concentrationin the container was 700 ppm.

[0108] This poly(vinylpyrrolidone) was subjected to the same heatingtreatment as of Example 28 together with the container. As a result, theK value of the poly(vinylpyrrolidone) was 89 after the heatingtreatment, therefore it was confirmed that the K value had changedlittle (reduction ratio=2.2%).

EXAMPLE 33

[0109] A poly(vinylpyrrolidone) powder was enclosed with nitrogen gas inthe same way as of Example 28 except that the powderypoly(vinylpyrrolidone) was replaced with a powderyvinylpyrrolidone/vinyl acetate copolymer (weight ratio ofcopolymerization=80/20) having a K value of 70.

[0110] This poly(vinylpyrrolidone) was subjected to the same heatingtreatment as of Example 28 together with the container. As a result, theK value of the poly(vinylpyrrolidone) was 70 after the heatingtreatment, therefore it was confirmed that the K value had not changed.

COMPARATIVE EXAMPLE 8

[0111] The glass container, in which the powdery poly(vinylpyrrolidone)having a K value of 91 was put, was set into a glove box in the same wayas of Example 28 and then airtightened without the introduction ofnitrogen gas into the glove box. At that time, the oxygen concentrationin the glove box was 20.95%.

[0112] This poly(vinylpyrrolidone) was subjected to the same heatingtreatment as of Example 28. As a result, the K value of thepoly(vinylpyrrolidone) reduced to 84 (reduction ratio=7.7%).

COMPARATIVE EXAMPLE 9

[0113] The glass container, in which the powdery poly(vinylpyrrolidone)having a K value of 91 was put, was set into a glove box in the same wayas of Example 28. Then, a mixed gas of nitrogen gas and air wasintroduced into the glove box for about 30 minutes to suppress theoxygen concentration in the glove box to 8%, and the glass container wasthen airtightened to enclose the powder of poly(vinylpyrrolidone) withthe above mixed gas.

[0114] This poly(vinylpyrrolidone) was subjected to the same heatingtreatment as of Example 28. As a result, the K value of thepoly(vinylpyrrolidone) reduced to 85 (reduction ratio=6.6%).

COMPARATIVE EXAMPLE 10

[0115] The glass container, in which the 20% aqueous solution ofpoly(vinylpyrrolidone) having a K value of 91 was put, was set into aglove box in the same way as of Example 29 and then airtightened withoutthe introduction of nitrogen gas into the glove box. At that time, theoxygen concentration in the glove box was 20.95%.

[0116] This poly(vinylpyrrolidone) was subjected to the same heatingtreatment as of Example 29. As a result, the K value of thepoly(vinylpyrrolidone) reduced to 85 (reduction ratio=6.6%).

[0117] Various details of the invention may be changed without departingfrom its spirit not its scope. Furthermore, the foregoing description ofthe preferred embodiments according to the present invention is providedfor the purpose of illustration only, and not for the purpose oflimiting the invention as defined by the appended claims and theirequivalents.

What is claimed is:
 1. A vinylpyrrolidone polymer, which contains atleast one antioxidant in a ratio of 0.00001˜30 weight % to thevinylpyrrolidone polymer.
 2. A vinylpyrrolidone polymer according toclaim 1, wherein the antioxidant is a compound having a phenolichydroxyl group.
 3. A vinylpyrrolidone polymer according to claim 1,wherein the antioxidant is a compound of general formula (1) and/or (2)below:

wherein each of R¹, R², R³, R⁴, and R⁵ is, independently of each other,at least one member selected from the group consisting of a hydrogenatom, alkyl groups, substituted alkyl groups, aryl groups, andsubstituted aryl groups, wherein R¹ and R², or R³ and R⁴, may be bondedto each other to form a cyclic structure.
 4. A vinylpyrrolidone polymeraccording to claim 1, wherein the antioxidant includes the joint use ofa radical scavenger with a peroxide decomposer.
 5. A vinylpyrrolidonepolymer according to claim 1, which further contains a radical formationinhibitor.
 6. A vinylpyrrolidone polymer, which is adjusted such thatthe reduction ratio of the K value defined by Fikentscher's equationwill be within 5% when the vinylpyrrolidone polymer is subjected to aforcible test in which the vinylpyrrolidone polymer is kept heated at120° C. under normal pressure in air for 2 hours.
 7. A vinylpyrrolidonepolymer, which is adjusted such that the reduction ratio of the K valuedefined by Fikentscher's equation will be within 5% when thevinylpyrrolidone polymer is subjected to a promotion test in which thevinylpyrrolidone polymer is kept heated at 50° C. in a solution statefor 14 days.
 8. A vinylpyrrolidone polymer according to claim 1, pH ofthe aqueous solution of which with a concentration of 5 weight % is inthe range of 4 to
 12. 9. A vinylpyrrolidone polymer according to claim3, pH of the aqueous solution of which with a concentration of 5 weight% is in the range of 4 to
 12. 10. A resin composition, which comprises avinylpyrrolidone polymer as a resin component, and further comprises atleast one antioxidant in a ratio of 0.00001˜30 weight % to thevinylpyrrolidone polymer.
 11. A stabilization process for avinylpyrrolidone polymer, which comprises the step of mixing thevinylpyrrolidone polymer with at least one antioxidant in a ratio of0.00001˜30 weight % to the vinylpyrrolidone polymer.
 12. A preservationprocess for a vinylpyrrolidone polymer, which comprises the step ofsuppressing the oxygen concentration to not higher than 50,000 ppm in agas phase that contacts with the vinylpyrrolidone polymer.
 13. Apreservation process for a vinylpyrrolidone polymer according to claim12, which further comprises the steps of putting the vinylpyrrolidonepolymer into an airtight container and evacuating the container.
 14. Apreservation process for a vinylpyrrolidone polymer according to claim12, which further comprises the steps of putting the vinylpyrrolidonepolymer into an airtight container and displacing air in the containerwith an inert gas and/or carbonic acid gas.
 15. A preservation processfor a vinylpyrrolidone polymer according to claim 13, which furthercomprises the steps of putting the vinylpyrrolidone polymer into anairtight container and displacing air in the container with an inert gasand/or carbonic acid gas.
 16. A preservation process for avinylpyrrolidone polymer according to claim 12, which further comprisesthe steps of putting the vinylpyrrolidone polymer into an airtightcontainer and enclosing at least one deoxidizing agent together with thevinylpyrrolidone polymer in the container.
 17. A preservation processfor a vinylpyrrolidone polymer according to claim 15, which furthercomprises the steps of putting the vinylpyrrolidone polymer into anairtight container and enclosing at least one deoxidizing agent togetherwith the vinylpyrrolidone polymer in the container.